The present invention relates to treatment fluids and compounds useful in subterranean formations, and, more particularly, to treatment fluids and compounds for retarding deposition of silica scale in subterranean formations. Treatment fluids can be used in a variety of subterranean operations, including, for example, stimulation treatments, conformance treatments, hydraulic fracturing treatments, acidizing treatments, remediation treatments, scale removal treatments, scale inhibition treatments, and the like. As used herein, the terms “treatment” and/or “treating” refer to any subterranean operation that uses a fluid in conjunction with achieving an intended function and/or an intended purpose. Use of these terms herein does not imply any particular action by the fluid or any particular component thereof. As used herein, the term “treatment fluid” refers to any fluid that can be used in a subterranean operation in conjunction with an intended function and/or an intended purpose.
Treatment fluids comprising an acidic base fluid can be used in a number of subterranean operations including, for example, stimulation operations and acidizing treatments. Treatment operations utilizing an acidic base fluid are especially challenging in some subterranean formations due to siliceous and aluminosilicate minerals commonly encountered therein. These silicon-containing minerals can interact with an acidic base fluid to produce dissolved silicon species, which can subsequently precipitate at higher pH values (e.g., greater than about 3) as amorphous, gelatinous and/or colloidal silica. As used herein, the terms “dissolved silicon” and/or “dissolved silica” will equivalently refer to silicic acid, silanols, and other soluble silicon species. As used herein, the term “silica scale” will refer to precipitated amorphous silica, precipitated gelatinous silica, precipitated colloidal silica, and hardened crusts of amorphous silica, gelatinous silica and/or colloidal silica.
Silicates (e.g., orthosilicates and metasilicates) are salts derived from silicic acid and other dissolved silicon species. As a class, silicates other than alkali metal silicates are sparingly soluble in water, particularly after polymerization to form polysilicates. Under mildly acidic to alkaline conditions, silicic acid monomers and other dissolved silicon species ordinarily condense into cyclic oligomers, which subsequently grow in size and eventually precipitate as colloidal, gelatinous and/or amorphous silica deposits (i.e., silica scale). A number of factors influence the saturation concentration of soluble silicon including, for example, pH, temperature, type(s) of dissolved silicon species present, ionic strength and the presence or absence of certain ionic moieties. For example, at pH values well below 1, dissolved silicon concentrations of 0.01 M or greater are attainable. However, at pH values of 3 or above, the saturation concentration becomes much lower. Certain metal ions, particularly Al3+, are especially adept at lowering the saturation solubility of dissolved silicon species even further. At pH values of 3 or above, Al3+ and soluble silicon species react to form insoluble aluminosilicate materials, thereby exacerbating an already challenging precipitation problem.
Various conditions can lead to the deposition and subsequent transformation of precipitated silica deposits into a hard crust of silica scale. For example, simple drying can transform precipitated silica deposits into a hard crust of silica scale. Silica scale buildup can form on any downhole surface such as, for example, tool surfaces and wellbore surfaces, which can detrimentally impact further subterranean operations. In addition, precipitated silica scale during an acidizing treatment can result in the plugging of pores in a subterranean formation, thereby decreasing porosity and detrimentally affecting yield.
Once formed, silica scale buildup can be difficult to remove. Silica scale buildup is typically removed through treatment with a strong acid. Although the saturation concentration of soluble silica increases with increasing acid strength, extremely acidic solutions can be unsuitable for certain subterranean formations. For example, subterranean formations having high pressures, high temperatures, and/or excessive quantities of acid-soluble minerals (e.g., sandstone) may not be effectively treated by using strong acids without the risk of undesirable formation damage occurring in some instances. In addition, the introduction of additional acids into a treatment operation can increase its cost and complexity.
Moreover, when the subterranean formation contains clays, which typically contain Al3+, the problem of silica scale removal can be exacerbated even further due to the aforementioned insolubility of aluminosilicate minerals. The removal of aluminosilicates also typically requires treatment with a strong acid, which has the undesirable effects mentioned above.
An alternative strategy for dealing with the problem of silica scale deposition in a subterranean formation is to suppress the deposition of amorphous, gelatinous and/or colloidal silica that leads to silica scale buildup. One way for suppressing the deposition of silica scale is to add a silica scale control additive to a treatment fluid that slows or prevents the polymerization of soluble silica species into precipitated colloidal, gelatinous and/or amorphous silica. Illustrative silica scale control additives that have been used in the art include, for example, phosphonates, aminocarboxylic acids, and polyaminocarboxylic acids. These agents are most effectively used at circumneutral and sometimes higher pH values (e.g., pH>˜5.5) due to the necessity of forming a deprotonated species for complexing the soluble silica. Certain polymeric species have also been used for inhibiting the deposition of silica scale. Illustrative polymeric silica scale control additives are disclosed in U.S. patent application Ser. No. 12/917,167, filed Nov. 1, 2010and now available as U.S. Patent Application Publication 20110079392, which is incorporated herein by reference in its entirety.
In acidizing treatments not utilizing silica scale control measures, dissolved silicon is often left to precipitate as the pH gradually rises from acidic to circumneutral conditions. Conventional polymeric silica scale control additives are typically used in this circumneutral pH region to address the deposition of silica scale after oligomerization of dissolved silicon has already occurred. To this end, conventional silica scale control additives are typically infused continuously to a subterranean formation over a period of hours to weeks to inhibit the deposition of silica scale. However, conventional silica scale control additives fail to address the short term oligomerization processes that ultimately lead to silica scale buildup.